Synthetic charge for material reduction mills



R. TURNER 3,058,675

SYNTHETIC CHARGE FOR MATERIAL REDUCTION MILLS Oct. 16, 1962 Filed Feb. 18, 1960 FIG.|.

FIG.3.

INVENTOR ROBERT TURNER ATTORNEYS.

United States Patent 3,058,675 SYNTHETIC CHARGE FOR MATERIAL REDUCTION MILLS Robert Turner, Toronto, Ontario, Canada, assignor to Aerofall Mills Incorporated, Columbus, Ohio, a corporation Filed Feb. 18, 1960, Ser. No. 9,518 Claims priority, application Canada Mar. 5, 1959 4 Claims. (Cl. 241-182) This invention relates to the reduction of materials in combined crushing and grinding mills, and more particularly it relates to the employment of a synthetic mill charge whereby the performance of such mills may be substantially improved.

In David Wetsons Canadian Patent No. 522,803, a mill structure for combined dry crushing and grinding mills is disclosed in accordance with which a mill having a diameter length ratio of more than 2:1 and provided with circumferentially spaced highly upstanding crusher bars about the interior periphery thereof is provided with opposed ring-like liners presenting a frusto conical surface to the periphery of the mill to effect a keying action on the charge in the mill. The principle of the keying action applies regardless 'of whether the mill is operated as a dry reduction unit or as a wet reduction unit, the employment of the keying action in relation to wet milling being described in David Westons U.S. patent application Serial No. 792,025, filed February 9, 1959, now Patent No. 3,010,661. Regardless of whether the mill is operated wet or dry, the effect of the keying action is basically that the keying liners act upon the larger pieces of material in the feed in such a way that the charge is maintained in more compact condition whereby useful reduction work may more efiiciently be transmitted to it.

In David Westons prior Canadian Patent No. 489,563, it was shown how the crushing action of dry combined crushing and grinding mills could be enhanced by adding to the mill charge a small quantity (up to about 3.5% of the mill volume) of large balls which would tend to concentrate in the toe of the charge and act as inertia bodies. Larger volumes of balls would tend to cause a too rapid breakdown of the larger pieces of feed bringing about a fall in efiiciency as the volume of balls was increased.

The effectiveness of the keying action in wet or dry combined crushing and grinding mills depends upon there being a sufiicient volume of large pieces of feed material within the mill during operation, and consequently there have been many types of material which did not lend themselves to reduction in combined crushing and grinding mills. Typically such refractory materials are those which already have a particle size which is too small to enable effective keying to take place or those materials which in their natural occurring state break down very readily to a comparatively small grain size substantially larger than the final product desired.

1 have now found that combined crushing and grinding mills of the type described may be made to operate very eifectively if there is employed what is herein referred to as a synthetic mill charge composed of reduction media and conforming to critical dimensional limitations calculated to maintain an effective keying action during operation of the mill.

In order that the dimensional limitations relating to improved novel synthetic mill charge may be clearly understood, use is made throughout the specification and claims of the expressions keying zone and effective mill volume, which are defined as follows:

Effective mill volume is the volume of a cylinder having a diameter equal to the interior diameter of the mill and having a length which is equal to the interior length of the mill measured at the periphery thereof.

Keying zone is the space between the circular apices 3,058,675 Patented Oct. 16, 1962 ice of the keying liners and the interior of the peripheral shell of the mill. It is mathematically derived by subtracting from the effective mill volume the volume of a cylinder having a diameter equal to the diameter of the circular apex of a keying liner, and a length equal to the interior length of the mill measured at the interior periphery thereof.

The keying liners which effect the keying action are covered with great particularity in U.S. Patent No. 2,704,636 (issued March 22, 1955) wherein FIG. 4 of that patent is reproduced as FIG. 1 hereof and FIGS. 2 and 3 of the same patent likewise reproduced herein as FIGS. 2 and 3, respectively. The invention will be explained in more detail in connection with the aforementioned FIGS. 1 to 3 accompanying the description wherein:

FIG. 1 is a diagrammatic view of the interior of the drum of a mill disclosed in FIG. 4 of the U.S. Patent No. 2,704,636 illustrating the general distribution and paths of travel of particles of material undergoing reduction when the drum is being rotated at a speed of about 82 to 92% of critical speed with the normal charge volume of this type of mill, i.e. about 20 to 32% of the drum volume;

FIG. 2 is a diagrammatic longitudinal section of a mill showing one form of annular keying liner; and

FIG. 3 is a diagrammatic longitudinal section of a mill showing another form of an annular keying liner.

Referring to FIG. 1, a mill of the type aforementioned is illustrated diagrammatically showing the general distribution of particle sizes within the mill and their paths of travel. As pointed out in column 3 of Patent No. 2,704,636, the body of the charge will be sitting up against the rising side of the drum bounded approximately by the heavy dotted line a, with toe b situated in the region of a plane passing vertically through the axis of rotation of the drum. Material fed to the mill will fall into the bottom of the drum just in front of the toe of the charge, and will form a false toe as illustrated. As the crusher bars advance into the charge, they will crush the pieces of material in the false toe 0 against the toe b' backed by the rest of the body of the charge, forcing them, as they are being reduced by the crushing action, through the true toe b and into the main body of the charge. Here, due to the well-known induced couple which makes all bodies within a mill tend to rotate on an axis parallel to the axis of the mill, the particles of material will rotate on their own axes and abrade one on the other, whereby they are further reduced by grinding.

As the mill operates, the general distribution of particle sizes within the mill and their path of travel will be substantially as illustrated in FIG. 1.

Surrounding the inlet and outlet openings of the mill and secured to the end walls thereof are annular keying liners of frusto-conical shape, e.g. annular members 27 and 28 together with 31 and 32 shown in FIG. 3 and 47 and 48 of FIG. 2, having deflecting surfaces 29 and 30 facing the axis of rotation of the mill illustrated by FIG. 1 of Patent No. 2,704,636. The provision of frustoconical surfaces 33 and 34 on annular members 31 and 32 of FIG. 3 or 47 and 48 of FIG. 2 has the eifect in bringing about a keying action on the charge.

Fines in thefeed, and oversize returning through the outlet of the mill will flow over the deflecting surfaces 29 and 30 (FIG. 2), being carried slightly around by their rotation, and deposit on the top of the cascade zone 31 and 32 and their frusto-conical surfaces 33 and 34 is two-fold. Firstly, as the material in the charge moves away from the periphery, the frusto-conical surfaces 33 and 34 react upon them in a substantially normal direction as indicated by the arrows 40 and 41 in FIG. 3. This reaction is carried from particle to particle in a manner tending to form bridges between the two surfaces 33 and 34 resisting the movement of material towards the axis of the drum. The result is that the charge is held together as a more compact mass and the particles are enabled to do more grinding work one on the other as they move relative to one another in the manner previously explained.

Thus when the crusher bar moves into the charge, the charge becomes momentarily keyed as a solidified, compact mass against which the material driven by the crusher bar is crushed, as against the stationary jaw of a jaw crusher. Moreover, the great force momentarily tending to break the bridges formed in the material as aforesaid as each crusher bar drives into the toe of the charge produces a secondary crushing action along the span of the bridges, causing these bridges to continually break and reform. In the result therefore considerably more useful energy can be put into the mill, and the capacity of a mill of any given size is increased.

It should be explained that the bridging referred to above actually takes place over considerable radial depth, for at its deepest point 42, the charge will actually cover the radially outward faces 43 and 44 of the annular elements 27 and 28 and bridging will occur between them. The zone where bridging is occurring will therefore extend radially from just inwardly of the crusher bars to just outwardly of the apices 45 and 46 of the annular members 27 and 28.

In cases where the mill is designed to treat a relatively fine feed material, the annular members 27 and 28 may be formed integrally with the annular members 31 and 32 in the manner illustrated in FIG. 2, with the vertical annular flats 47 and 48 extending between the deflecting surfaces 29 and 30 and the frusto-conical surfaces 33 and 34. Once initiated by the surfaces 33 and 34, the bridging zone will extend upwardly between these flats, the shorter distance between the annular flats 47 and 48 permitting the bridging of smaller particle size material than would bridge satisfactorily in the arrangement shown in FIG. 3.

In order to bring about satisfactory bridging conditions and provide the keying action explained above, certain dimensional limitations are imposed on the design and arrangement of the annular members taking part in the action. Firstly, the apices of the annular members carrying the frusto-conical surfaces 33 and 34 must each extend inwardly into the mill interior a distance not less than of the mill length. They may extend inwardly up to about 25% of the mill length but it is not recommended that they be designed to extend inwardly appreciably more than about of the mill length because the volume of mill occupied by the annular members 31 and 32 will in that case begin materially to reduce the volume available in the mill for charge. Practical experiments have indicated that to obtain a maximum of keying effect Without appreciably reducing the space in the mill available for charge, annular keying elements which extend inwardly about 15% of mill length are to be preferred.

Secondly, while the angle of inclination of the frustoconical surfaces 33 and 34 is not critical, it is preferred to have them incline at an angle of about 120 to the end walls of the drum.

Finally, the relationship between the distance m1 between the opposed apices of the annular members 31 and 32 and the distance m2 from these apices to the crusher bar faces is most important, and for good results is selected so that the ratio lies between 0.4 and 1.5.

Depending upon the particular material which is being reduced and the particular grind which is required as a product, the keying zone in combined crushing and grinding mills will vary within the range of from about 30 to about 70% of the effective mill volume. In general, in mills of this type a keying zone which is about 50% of the effective mill volume is found to be satisfactory for most purposes in dry combined crushing and grinding mills whereas in wet mills of this type, it will be usual to design the mill with a slightly smaller keying zone than would be used in the dry mill.

Combined crushing and grinding mills which are designed to provide a keying zone in the manner above discussed have three principal characteristics which make their use particularly advantageous compared to the use of conventional milling equipment. Firstly, of course, such mills may be used to reduce material from a run-ofmine or primary crusher product to a final product size range in a single unit operation. Secondly, in general it has been found that where the feed material lends itself to the maintenance of a properly keyed charge, the power consumption per unit of product is substantially less than in the case of conventional equipment. Thirdly, the action of such mills tends to produce a product which is more amenable to metallurgy in that the mineral constituents are generally freed in their natural grain sizes and there is a minimum of overgrinding or sliming.

In the past, it has been believed that materials in which there were no lumps of greater size than about 3 or in which such lumps where they did occur were subject to very rapid breakdown in the mill were not capable of eflicient reduction in combined crushing and grinding units of this nature. Such materials in general could not be treated at an economic rate of production and because of the absence of an effectively keyed charge would tend to produce an uncontrollable amount of slimes.

It had previously been noted particularly in the reduction of hard, tough material Where the feed consisted largely of large lumps of material that where a small number of large balls, e.g. up to about 3.5% of the mill volume had been added as inertia bodies to increase the rate of breakdown of large particles of feed, that further additions of balls tended generally to reduce the capacity of the mill due to loss of keying action brought about by the absence of sufiicient large lumps of material persisting in the charge to provide for keying and it was consequently believed that substantially larger additions of balls would merely serve further to reduce the mill capacity.

I have now found, however, when a sufficient volume of reduction media is added to such mills to enable the charge of reduction media itself to engage the keying liners to produce a keying action in the ball or reduction media charge that very substantial increases in capacity are made possible, and moreover that the capacity and efficiency of such mills can be maintained where there are little or no large lumps of material in the feed. Instead of balls consisting of forged steel or other conventional materials such as are usually used as reduction media, a corresponding volume of lighter weight metal or rock may be used with like effect, the individual media used being either round or rounded in shape or being of any other shape. Thus, the object is to produce synthetically a keying effect comparable to that which is produced in such a mill when operating with materials which naturally produce a keyed charge in the absence of reduction media.

According to the invention, a charge of reduction media is maintained in a combined crushing and grinding mill of the type described which occupies a volume equal to from about 10% to about 45% of the keying zone of the mill. The reduction media employed are preferably graduated in size, and suitable for many applications is a synthetic charge consisting of conventional steel balls graduated in size from about 1" to about 6". In many applications where it is desired to relatively alter the crushing forces with respect to the grinding forces to suit specific materials, use may be made of higher specific gravity reduction media such as for instance media composed of tungsten carbide or other high gravity materials. On the other hand, in many cases it is desirable to employ lighter reduction media composed of metals lighter than iron, or to use a synthetic charge which consists of a pebble charge of material having a specific gravity not materially greater than that of the mill feed. In addition, as will be appreciated, some materials in the form in which they are to be reduced may contain some large lumps but not sulficient to effectively produce the keying action contemplated by the present invention. In such cases, these large lumps may he used as reduction media to produce a synthetic charge consisting of these large lumps, together with an additional quantity of selected pebbles or other reduction media.

While reference has been had to grinding balls as the reduction media, it will be appreciated that a spherical shape is not necessarily the shape best adapted to produce results in all circumstances, and consequently the present invention contemplates the use of reduction media of shapes other than spherical.

What I claim as my invention is:

1. In a material reduction mill comprising a drum defined by two end walls and a cylindrical wall mounted for rotation on a substantially horizontal axis, said drum having a diameter length ratio of at least 2:1 and highly upstanding transverse crusher bars spaced apart around the interior periphery thereof sufiiciently wide apart effectively to engage the largest pieces of material to be fed thereto, said drum also being provided with aligned axial inlet and outlet ports and having surrounding said ports on the end walls thereof circular keying liners of frustoconical shape having apices extending inwardly of said end walls so as to define a keying zone in the space between said inwardly extending apices and the interior periphery of the mill in an amount ranging from about to 70% of the efiective mill volume, the combination of a feed charge and a synthetic charge of reduction media, said synthetic charge having a volume inclusive of voids falling within the range of 10% to about of the volume of the keying zone, the size of the reduction media constituting the synthetic charge being sufiicient to promote keying of the feed charge within the keying zone.

2. The apparatus defined in claim 1, wherein the synthetic charge is composed of grinding balls graduated in size from about 1" to about '6 in diameter.

3. The apparatus defined in claim 1, wherein the synthetic charge is a pebble charge.

4. The apparatus defined in claim 1, wherein the synthetic charge is composed of metallic reduction media of non-spherical shape.

References Cited in the file of this patent UNITED STATES PATENTS 2,680,568 Weston June 8, 1954 2,704,636 Weston Mar. 22, 1955 2,824,700 Weston Feb. 25, 1958 

